The Bombers coach released a written statement after a marathon meeting in Melbourne's Docklands precinct.
"At the interview I fully co-operated and truthfully answered every question that was put to me by the investigators," Hird said.
"I will not do anything to impede the ASADA and AFL investigation and will continue to co-operate fully.
"This includes maintaining the confidentiality of the interview. This is incumbent upon all parties present at the ASADA and AFL investigation so as to not pollute the process.
"I welcomed the opportunity to tell the truth today."
Hird left Etihad Stadium just after 5.30pm after spending the day with investigators. He was accompanied by his legal team.
It's not yet clear if he'll be required to attend further talks on Wednesday.
While tied up with the interview, he missed the Bombers training session at Tullamarine on Tuesday morning.
Hird said prior to the interview that he was "looking forward" to having his say.
He attended the meeting with his legal representative Tony Nolan SC.
It's believed ASADA had slated up to three days to speak with Hird, but it's unlikely he'll be with them for all of that time. The interview will be recorded to be used as evidence.
Two ASADA investigators are conducting the interview while an AFL integrity unit representative is also believed to be in attendance.
Hird is expected to have gained a clear sense of the case being pursued against his club.
It's possible ASADA has presented the Bombers coach with hard evidence from its inquiries so far, giving the club a real insight into how the doping investigation has progressed.
"This is actually the start of the process ... there will likely be multiple interviews and re-interviews," former ASADA boss Richard Ings told AFL.com.au on Monday night.
"We're up to about chapter two of a 20-chapter book."
Ings said the format of the interview would be straight out of the government handbook, and conducted similarly to a customs or therapeutic goods administration inquiry.
Hird will be free to publicly discuss details of his interview afterwards, but AFL.com.au understands he has no plans to do so until the investigation is completed.
"While ASADA is bound by confidentiality as to the contents of the interview, the subject of the interview has no such confidentiality provisions," Ings said.
"They can speak about the interview as they see fit to speak."
Essendon assistant coach Mark Thompson said Hird was coping with the pressure.
"We can't talk about that," Thompson said of Hird's ASADA meeting.
"He needs support and care and he goes up and down, but he's still coaching well and he's strong.
"He's a strong man and he's very, very determined.
"He's doing his job - he's doing what he has to do - and he'd be a pretty busy guy, I'd reckon."
He added the crisis was affecting Hird, telling the Fox Footy: "He has moments, but he's been focused."
Thompson compared it to 2006, when he nearly lost his job as Geelong senior coach.
"I went through a similar thing in 2006 and luckily I got a reprieve - someone forgave me for making a few errors," he said.
"He has been coaching well and it's important he's coaching well.
"If we keep coaching well and the team keeps playing well and we find a way to get through what we're getting through and we're still able to do our job well, then that says a lot for our footy club."
Thompson said he was helping Hird "a little bit".
"He just needs help, he needs support."
The ASADA interview process is likely to take weeks, with players and club officials also set to be interviewed.
A separate, independent investigation being conducted by former Telstra boss Ziggy Switkowski is believed to be well advanced, and likely to be handed to the Bombers board in the next two to three weeks.
Matt Thompson is a reporter for AFL Media. Follow him on Twitter @MattThompsonAFL